Conventionally, defect detection on semiconductor wafers can be performed with either optical or electron beam inspection. Systems and methods for inspecting semiconductor wafers for defects using optical and electron beam inspection techniques are generally well known.
Optical inspection systems frequently use either microscopic type imaging or the collection of the scattered energy. For the microscopic type of optical inspection, it may be difficult to inspect defects that generate little intensity change from the nominal structures. For example, dark defects on a dark background are typically difficult to detect due to the closeness of the change in intensity in the reflected image due to the dark defect on the dark background.
Conventional systems that collect optical images of a given substrate can be generally divided into two categories depending upon the method by which they obtain an image of a given area, namely (1) area imaging systems and (2) scanning systems. In area imaging systems, a whole area of the substrate is illuminated at once and imaging optics are used to project an image of that area or a part of it upon a detector array, such as a charge coupled device (CCD) camera. In scanning systems, however, a spot, rather than an area, is illuminated and scanned upon the substrate, and the transmitted or reflected light is measured by one or more detectors either directly or after passing through collection optics. The illumination beam may be scanned across the surface in both directions or in just one direction with mechanical motion of the substrate relative to the beam used to obtain the two-dimensional area image.
In scanning systems, illuminating light is focused upon a small spot of the substrate to be imaged and is moved across the substrate in one or two dimensions. Some of the light that is reflected or scattered from the spot is collected upon at least one detector, which is sequentially sampled. The detector's output along with the knowledge of the location of the spot location at any given time is used to reconstruct an image of the area scanned.
Area imaging systems and scanning systems have relative advantages and disadvantages. For example, one disadvantage of scanning systems is their serial, rather than parallel, nature. Hence, it typically takes longer to construct an image using a scanning system than an area imaging system. An advantage of scanning systems over area imaging systems, however, is the ability to use laser sources that have both a high brightness and a potentially narrow spectral emission range. The latter may be particularly important for UV optical systems where it is difficult to correct for spectral aberrations.
Besides the division between area illumination-based systems and laser-spot scanning-based systems, imaging systems are also divided by the direction of the illumination with respect to the collection optics. In general, if the illumination impinges, or is incident, upon the substrate from a direction such that the specularly transmitted or reflected light is collected by the imaging optics and then detected, the system is termed “bright field” (“BF”), and the detectors are known as bright field detectors. If, on the other hand, the illumination arrives from a direction which is outside the collection angle of the collection optics for the detector(s), the system is termed “dark field” (“DF”).
Dark field imaging is typically used to enhance edge phenomena by collecting only the scattered light. When used for optical inspection, dark-field laser scanning systems greatly improve the signal to noise ratio for small, three-dimensional objects in a mostly flat background. Furthermore, using several dark field detectors located in different angles within the dark field may increase the chance of defect capture.
It has been found that, in some applications, defect detection can be improved by using phase detection rather than intensity based detection, because defects that create little intensity or little intensity change typically have a modest phase signal.
One system for defect detection using phase detection is disclosed in U.S. Pat. No. 6,078,392, which is incorporated herein by reference in its entirety. This patent proposes a direct-to-digital (DDH) holography approach wherein a reference beam is incident upon a reference beam mirror at a non-normal angle, and the reference beam and an object beam are focused at a focal plane of a digital recorder to form an image. This direct-to-digital holography approach, however, requires significant computational power, which may limit throughput. In addition, this approach is limited in that it does not provide for collection of scattered energy.
Additional background details are disclosed in U.S. Pat. No. 6,122,046 the disclosure of which is expressly incorporated herein by reference.